Partial-Beam
Filling - a limitation of the rainfall
estimation techniques used by NEXRAD.
At far ranges from the radar, a storm
may occupy only a portion of the radar beam (which may be several miles
across). However, the radiation received
by the radar antenna consists of the average reflectivity
across the entire beam, so the reflectivity and associated rainfall rates
are underestimated.

Perigee
- the point in the moon's orbit when it is closest to the earth;
opposite of apogee

Perihelion
- the point in the earth's orbit nearest the sun (147 million
kilometers from the sun), on about 3 January; opposite of aphelion

Period
- (1) the time interval for a planet or satellite to complete
one revolution on its orbit or (2) the time interval between passages,
at a fixed point, of the crest of a wave (e.g.,an electromagnetic wave).
Mathematically, it is the reciprocal of frequency.

Permafrost
- a permanently frozen layer of soil or bedrock at a variable
depth below the surface of the earth in frigid regions

Persistence
Forecast - a forecast that the current weather condition will
persist and that future weather will be the same as the present

Polar
Orbiting Satellite - a satellite with an orbit nearly parallel
to the earth's meridian lines which crosses
the polar regions on each orbit

Positive
Area - the area on a sounding
representing the layer in which a lifted parcel would be warmer than the
environment; thus, the area between the environmental temperature profile
and the path of the lifted parcel

Positive-Tilt
Trough - an upper level system which is tilted to the east
with increasing latitude (i.e., from southwest to northeast); often it
is a sign of a weakening weather system

Potential
Temperature - the temperature
a parcel of dry
air would have if brought adiabatically (i.e., without transfer of
heat or mass) to a standard pressure level of 1000 mb

PPI
- an acronym for Plan Position Indicator. A PPI displays radar
data horizontally using a map projection. In PPI mode, the radar makes
a 360-degree sweep with the antenna at a specific elevation angle. A PPI
display is the familiar radar display shown on the television weather
programs.

PPINE
(Plan Position Indicates No Echoes)
- referring to the fact that a radar
detects no precipitation within its range

Precipitation
- any form of water particles, either liquid
or solid, which falls from the atmosphere
and reaches the ground

Precipitation
Mode - the standard, or default, operational mode of the WSR-88D.
The radar automatically switches into precipitation
mode from clear-air mode if the measured
reflectivity exceeds a specific threshold
value. The precipitation mode of NEXRAD is
more sensitive than previous weather radars. The minimum detectable reflectivity
in NEXRAD's precipitation mode is 5 dBZ, compared to 28 dBZ with the old
WSR-57.

Pressure
- a force per unit area or a stress characterized by uniformity
in all directions

Pressure
Gradient - the change in pressure over
a given distance at a given time

Pressure
Tendency - the change in pressure over
a given time at a given location

Prevailing
Wind - the wind direction most commonly
observed during a given period

Probability
- the chance that a given event will occur

Probability
Forecast - a forecast of the probability that one or more of
a mutually exclusive set of weather conditions will occur

Probability
of Hail - a product from the NEXRAD
hail detection algorithm that estimates
the likelihood that hail is present in a storm

Probability
of Severe Hail - a product from the NEXRAD
hail detection algorithm that estimates
the likelihood of hail exceeding 3/4 inch in
diameter is present in a storm

Profiler
- an instrument designed to measure horizontal winds
directly above its location, and thus measure the vertical wind profile.
Profilers operate on the same principles as Doppler
radar.

Propagation
- the movement of an atmospheric phenomenon. This term is typically applied
to the motion of thunderstorms into
regions favorable for their continued development (into a maritime
tropical airmass).

Psychrometer
- an instrument used for measuring the water
vapor content of the atmosphere; a hygrometer
consisting essentially of two similar thermometers
with the bulb of one being kept wet so that the cooling that results from
evaporation makes it register a lower
temperature than the dry one and with
the difference between the readings constituting a measure of the dryness
of the atmosphere

PUP
- an acronym for Principal User Processor. A PUP is the hardware
component of the NEXRAD system that consists
of a user workstation with two graphics monitors and a graphics tablet.
The PUP receives NEXRAD products from an RPG,
and the user can interact with the products using the PUP.

PVA
(Positive Vorticity Advection) - the
advection of higher values of vorticity
into an area; often PVA is associated with upward motion (lifting) of
the air